1. Field of the Invention
This invention concerns electrically controlled panes of the electrochromic type, that is to say panes in which the coloration can be modified by an electric field.
2. Discussion of the Background
The operating principle of electrochromic panes is based upon the use of a film of a material having an electrochromic property, which is capable of accepting the insertion of cations in a reversible manner, notably H.sup.+ protons or lithium cations Li.sup.+, thereby passing the film from one degree of oxidation to another, it being well known that each of the degrees of oxidation corresponds to a different coloration state.
For example, tungsten trioxide, virtually colorless in its oxidized state (a) acquires a strong deep blue color in its reduced state (b) by the insertion of protons or Li.sup.+ cations according to the following reaction: ##STR1## To make these reversible changes of color possible, electrochromic panes comprise two glass substrates, between which are disposed two transparent electrically conducting films equipped with current supply leads, separated successively by an electrode constituted of an electrochromic material, termed a cathodic material, such as tungsten oxide, a counter-electrode, which may also be constituted of an electrochromic material termed anodic, such as iridium oxide or nickel oxide, and a film of electrolyte which makes possible the transfer of the cations in question from one electrode to the other.
By using iridium oxide as a counter-electrode, this material, in contrast to the electrode, is colorless in the reduced state (b) in which H.sup.+ protons are inserted and is grayish yellow in the oxidized state (a), where it is deprived of H+ protons, which reinforces the contrast in coloration of the pane, in accordance with the following reaction: ##STR2## A counter-electrode based upon nickel oxide may, for its part, either insert protons just like iridium oxide, or insert lithium cations, as is known from Patent Application EP-A-0 373 020, with a more complex reaction arrangement implying the existence of several electrochromic equilibrium states.
To express it simply, by applying a potential difference of a desired value and polarity to the two electrically conducting films, a cationic transfer takes place through the electrolyte from one electrode to the other, causing an insertion or de-insertion reaction of the cations and thus a modification to the overall colored appearance of the pane. The perfecting of an electrical supply system is thus crucial for the proper functioning of such a pane. Thus, for example, European Patent Application EP-A-408 427 proposes a supply system which optimizes the switching times of the electrochromic material by providing three current leads in an arrangement of the potentiostat type and applying a potential difference which can be programmed and varied as a function of time. Switching time is to be understood here as meaning the time that elapses during the passage from one given coloration state to another of greater or lesser coloration than the starting one. In this way it is possible to compensate the phenomenon of ohmic drops which appear in the electrically conducting films and are difficult to prevent in the case of large panes.
This document therefore provides for a modulation of the applied potential difference, which enables the degree of coloration and switching time desired to be controlled.
Panes of this type are especially interesting for equipping buildings or vehicles in order to control the solar input and their envisaged applications are tending to diversify widely. A new problem, however, now arises: the proper functioning of the electrochromic pane essentially requires a correct and properly controlled electrical supply.
In the case of a malfunction of the electrical supply, for example in the case of an accidental disconnection of one of the electrical supply leads or even a crack in the pane or a detachment of screen printing, if the electrochromic material is then relatively or highly colored the operator no longer is able to cause it to return to the colorless or slightly colored state within a short period of time. The operator can only wait for a self-discharge phenomenon of the system, a phenomenon which, happily for the kinetics, is extremely slow, taking at least several days, which tends to bring the electrochromic material of the electrode back into its equilibrium state, which corresponds to its almost completely colorless state. This self-discharging can have various causes, more or less well understood and intrinsic to the nature of the films used in the stack of the electrochromic system: it can conceivably be assumed that the electrolyte film, which is conducting for cations, is not a perfect electron insulator, for example. It is also possible to think of weak short-circuits between the electrically conducting films.
Now it may well prove to be important to be able to get back as rapidly as possible to the colorless state or virtually colorless state in the case where electrical supply fails. This is especially true when security may be at risk, when these electrochromic panes are to be fitted to vehicles, and notably to aircraft cockpits or drivers' cabs of railway locomotives, or even when these panes are used as rearview mirrors for automobiles.